Illustrated in FIG. 12 is a conventional hydraulic circuit of an infinite variable-speed hydraulic drive vehicle which has a variable capacity hydraulic pump 1 and a motor 2. In the same figure, a part of the output of an engine 5 drives a working machine hydraulic pump 4 and acts upon a working machine hydraulic cylinder 52 by way of a working machine hydraulic circuit 51. Remaining output of the engine 5 drives the control pump 3 and a hydraulic pump 1 in which oil pressure is generated. The oil pressure thus generated at the hydraulic pump 1 flows through main circuits 24 and 25, rotates the variable capacity motor 2, thereby forming a driving force.
Designated at 6 is a pump control valve for controlling the capacity of the hydraulic pump 1, 7 is a pump capacity control cylinder, 8, 8 are main relief valves and 10 is a filter. Oil under pressure, which flows from the pump control valve 6 to the motor control oil line 11, is introduced into one end of the motor control valve 14, whereby the oil introduced into the motor control valve 14 operates to introduce the oil under high pressure, which is introduced by a pilot hydraulic piping 16 from the main circuits 24 and 25, into a motor capacity control cylinder 31.
That is, the pump capacity control cylinder 7 and the motor capacity control cylinder 31 are controlled by the pump control valve 6 and the motor control valve 14 so that the capacity of the pump 1 and the capacity of the hydraulic motor 2 are arbitrarily varied, thereby varying the traveling speed of the vehicle.
A graph representing a traveling performance curve of the conventional hydraulic drive vehicle, as explained in FIG. 12, will be described with reference to FIG. 13. As illustrated in the same figure, in the infinite variable-speed hydraulic drive vehicle having the conventional variable capacity hydraulic motor, both the traveling driving force and the traveling speed are continuously varied so that they are automatically and continuously varied without changing operation from the maximum driving force (traveling speed is 0) to the maximum speed. Accordingly, since the driver can control the traveling speed and the driving force, it is possible to travel the vehicle easier than a vehicle provided with a mechanical speed change gear.
The conventional hydraulic drive vehicle has an advantage that the traveling operation is easy. However, in the industrial vehicle provided with a loading apparatus such as a shovel loader, the working machine pump 4 of the loader is also driven by the engine in the same way as the hydraulic drive pump 1. Since the maximum rising speed of the loader is proportional to the engine speed, the rising speed of the loader is also controlled by the travel of the accelerator pedal in the same way as the traveling speed. Accordingly, in case that the hydraulic drive vehicle travels while raising the loader at the working site, the working efficiency is improved, when the loader rises at the maximum speed and the vehicle travels at low speed. However, there was such a serious problem that when the loader rises at the maximum speed, the traveling speed at the same time increases at the maximum. This problem will be described with reference to FIG. 14 which depend on the engine speed of the engine 5. Pst1", . . . Pst5" represent pressures of pilot hydraulic piping 15. The relation between the capacity of the motor (horizontal axis) and the pressure P.sub.H" (vertical axis) in the main hydraulic circuits 24 and 25 is varied by the pilot pressures Pst1" . . . Pst5". In case that the traveling speed is accelerated at full throttle so as to rise the loader at the maximum speed, as the pilot pressure rises to Pst1" and thereafter the traveling load decreases, the pressure P.sub.H " decreases as the traveling load decreases. At this time, the traveling speed increases as the capacity of the motor moves along the characteristic line of the pilot pressure and reaches the minimum. At this time, the traveling speed reaches the maximum speed, which causes a problem at the working site having a small traveling range.
To solve the matching problem between the traveling speed and the rising speed of the loader, the multiple speed gear change method is conventionally employed wherein the maximum traveling speed is reduced while the capacity of the motor is fixed to be the maximum or the mechanical speed change gear is connected to the motor. However, there was a problem in the former in that the traveling speed is limited to the low speed since the capacity of the motor is fixed as illustrated in FIG. 15 and a problem in the latter in that the device cannot match exactly with the working condition since the maximum driving force is not obtained at high range as illustrated in FIG. 16. To solve these problems, the applicant proposed a means, which is disclosed in Japanese Utility Application No. 63-144985 (and corresponding U.S. Pat. No. 5,092,153) and is provided with a means which is capable of operating to control the minimum capacity of the variable capacity hydraulic motor, thereby continuously controlling the maximum traveling speed in response to the oil pressure in the high pressure side of the main circuit which is connected to the variable capacity hydraulic motor of the hydraulic pump and the pilot oil pressure from the control pump which operates the motor capacity control cylinder for controlling the capacity of the variable capacity hydraulic motor. A motor control valve 14 of this device, as disclosed in Japanese Utility Model Application No. 63-144985, controls the motor control oil pressure by balancing the control oil pressure with the main circuit oil pressure, but it had the following problems:
(1) The motor control oil pressure is liable to vary by the variation of the main circuit oil pressure which is caused by the control of the traveling speed.
(2) The structure is complex and rather expensive.
It is therefore an object of the present invention to solve these problems of the conventional device and to provide a device for controlling the variable capacity motor which is simple in structure and low in cost and has a traveling characteristic which is not influenced by the variation of the main circuit oil pressure.